🌪️ Dust devils
Tight cores of swirling wind will pick up dust, leaves and other debris to become a visible ground disturbance or towering column of brown dust in areas of bare ground. Such whirlwinds are known as willy-willies in Australia and dust devils elsewhere. Dust devils occur when a thermal lifts off in superadiabatic conditions. The air rushing in to fill the area below the thermal usually has some turning motion due to Coriolis effect if it has been flowing for some time. When this air comes together its spin is exaggerated just as a skater spins faster when his or her arms are brought in. This spinning air would soon lose its impetus except for the accelerating thermal "stretching" the air vertically and bringing the rotating column tighter as it gets higher, much like a column of thick syrup gets thinner as you pull the spoon out of it.
🌪️ How dust devils work
Dust devils are formed when thermals rise in a superadiabatic lapse rate. Dust devils lie under the rising thermal, mark its track, size and often height as well as duration.
Dust devils sometimes reach up into a thermal cloud, but usually stop well below this level, being typically only several feet to several hundred feet high (up to 100m). In some desert areas however, they can tower over several thousand feet (1,000 m) when fine dust and strong continuous thermals abound. In these areas the height of the dust devil will indicate the minimum height of the thermal as well as its duration. However, at times the dust devil lasts past the production of usable lift as many unhappy pilots diving for a devil have found out. Watching the climb altitudes and rates as well as the duration of dust devils helps you judge the duration of the thermals creating them. The vast majority of dust devils turn counterclockwise in the northern hemisphere and clockwise in the southern hemisphere. They are low pressure phenomena. The few devils that turn in the opposite direction are probably artifacts of rotation that began through turbulence or moving past a bluff. There is some conjecture that dust devil action spins the thermal air, and indeed, rotating thermal clouds have been seen on a rare occasion. It is likely that the air continues to spin above the dust although it probably stops its spin due to drag when the thermal leaves the superadiabatic layer. On this basis, it is reasonable to expect a better climb rate when turning against the flow of the dust devil (clockwise or to the right in the northern hemisphere) when in the strong lift of the superadiabatic layer. The reason for this better climb rate against the flow in spinning air is your rate of circling is slower so less bank angle is required to offset centrifugal force. Less bank angle gives you a better sink rate. It is also important, to enter a dust devil thermal going against the flow for safety reasons. If you join the spinning air in the same direction as the flow you will experience a sudden strong tailwind which may stall you. If you enter against the flow you will experience an increasing headwind, as shown in the diagram below, which will provide improved maneuverability. A dust devil is a stable entity in that air from the outside cannot join the dust devil along the column and dilute it. Outside air can only enter it from below where the spin is slowed close to the ground as shown in the diagram below. The air on the outside of the column is spinning and rising as shown while inside the column downward flow can occur due to lowered pressure. An example of this action can be seen in a stirred cup of coffee with up flow on the outside and a depression in the middle. The center of the dust devil is generally clearer than the sides. The death of a dust devil occurs when the supply of warm air feeding the thermal is exhausted or some terrain effect blocks its progress. Dust devils will of course move up a steep mountain and are in fact quite common on heated slopes. A dust devil may continue a bit past the life of the thermal, but the devil soon looses energy and collapses. Witch doctors in Africa had a good business destroying dust devils by running through them, leaving the populace in awe of their demon-defeating powers.
